1. Field of the Invention
The present invention relates to a method of in-situ formation of a reference electrode for in-tank use in plating bath analysis. More particularly, the present invention relates to a method for repeatedly regenerating a reference electrode to provide long-term reference voltage stability without replacing the electrode. The stable reference electrode is of the type ideally required in in-tank sensors used to accurately measure and analyze the electrochemical properties of plating baths.
2. Description of Related Art
Plating bath analysis methods, such as the method disclosed in U.S. Pat. No. 4,631,116, and assigned to the present common assignee, typically use electrochemical sensors containing sensing electrodes and a reference electrode. The sensing electrodes usually include a working electrode and a counter electrode. All of the electrodes are in direct contact with the plating bath solution. The electrochemical properties of the plating bath solution are measured by applying ac and dc signals to the solution via the sensing electrodes and measuring the resultant response signals. The reference electrode plays an important role in the proper functioning of the sensor, since it provides a reference voltage upon which the various ac and dc signals are applied and controlled at the sensing electrode during measurement.
The reference voltage is a function of the particular type of plating bath, and reflects changes in the bath that are independent of the applied signals. Basing response signal measurements on this reference voltage permits accurate monitoring of a variety of important electrochemical properties. Absent the reference voltage, normal ground-referenced voltage variations for a given plating bath will interfere with the control and response signals, preventing robust and accurate electrochemical analysis.
Commercially available standard reference electrodes are not suitable for in-tank use and suffer from the following limitations: frequent and time consuming maintenance, contamination of the plating bath, fouling, unsatisfactory stability, or structural characteristics not readily compatible with in-tank sensor designs.
Alternatively, wire reference electrodes with material matching the plating bath have been suggested for use. U.S. Pat. No. 4,812,210 describes various reference electrodes of this type. The wire may also be formed from a base metal substrate suitably plated for a particular application. Usually, the base metal is the same metal as the plating application or a less expensive material.
The reference electrodes described above suffer from a number of defects which render them inadequate for in-tank applications. The reference electrodes require costly and time-consuming maintenance, since they frequently wear out and must be replaced. Moreover, the plating on a plated reference electrode deteriorates over time, and can eventually flake off and contaminate the bath. The flakes can also accumulate in an electrochemical sensor and thereby interfere with the operation of the working and counter electrodes.
Furthermore, the currently utilized reference electrodes exhibit a voltage instability which is unacceptable in the many plating bath applications which require continuous operation for periods of several weeks or more. For example, tests performed on a solid copper wire reference electrode showed that the electrode voltage became unstable in less than 24 hours. This type of voltage instability significantly degrades the accuracy and efficiency of voltammetric analysis techniques such as those described in U.S. Pat. No. 4,631,116. A user requiring continuous analysis of the plating bath must either accept continually degrading measurement accuracy or interrupt production to manually replace the reference electrodes several times a day.
As is apparent from the above, there presently is a need for a stable reference electrode for in-tank use which does not require replacement or generate contaminants within the plating bath or sensor. Further, the reference electrode should provide a stable voltage reference during the electrochemical analysis and over long periods of time. The reference electrode should provide these features and also be compatible with most in-tank electrochemical sensors, plating baths and the measurement methods and equipment associated therewith.